Human activity can be classified based on comparisons between physiological and/or motion-related metrics and absolute thresholds. Sensors have been used to make measurements, based on which metrics are calculated. The current values of metrics are compared with absolute ranges to indicate whether adjustments are desired or needed. For example, a runner's heart rate metric might be compared with fixed thresholds and a warning produced if the metric varies outside a fixed range believed to be beneficial for training, competition, and/or recovery. In this case, the heart rate might be classified as inappropriately high, appropriate, or inappropriately low.
There exist methods of detecting signs of fatigue based on changes in heart rate. Such methods are unable to detect fatigue that results in deteriorating athletic form or technique due to changing properties of individual muscles.
There exist methods to measure physiological and/or motion-related data at fixed frequencies. High-frequency measurement results in high power consumption for battery-powered electronic systems, requiring large batteries or resulting in short battery lifespans.
There exist methods of measuring motion and position (and thus distance), such as via the Global Positioning System (GPS). These methods result in high power consumptions for battery-powered instantiations, limiting battery life or requiring large batteries. Existing methods of estimating changes in distance using lower-power sensors have been limited by error that increases with passing time and motion.
There exist methods of estimating the motion and positions of people engaging in bipedal motion, and the motion, positions, and orientations of their limbs. For example, Global Positioning System (GPS) technology can be used to determine the positions of people. However, this approach is prone to error in areas without access to satellite signal. Using this approach to estimate motion over short distance scales is error prone. In addition, this approach generally has high power consumption, resulting in short battery lifespans or large sizes for portable devices.
There are also techniques to estimate motion and positions of people using data from inertial measurement units fused with data from absolute or relative orientation measurement devices such as magnetometers or gyroscopes. Existing approaches are not capable of tracking positions and orientations of limbs for use in gait analysis. They are also prone to high power consumptions, resulting in short battery lifespans or large sizes for portable devices.